1. Field of the Invention
The present invention relates to an electrolyte material for a polymer electrolyte fuel cell, a liquid composition comprising the electrolyte material, and a membrane/electrode assembly for a polymer electrolyte fuel cell containing the electrolyte material in a catalyst layer.
2. Discussion of Background
As an electrolyte material contained in a catalyst layer of a membrane/electrode assembly for a polymer electrolyte fuel cell, the following polymer (1) has been known.
(1) A polymer having sulfonic acid groups (—SO3H groups) converted from —SO2F groups in a polymer having repeating units based on a compound represented by the following formula (m3) and repeating units based on tetrafluoroethylene (hereinafter referred to as TFE):CF2═CF(OCF2CFZ)mOp(CF2)nSO2F  (m3)wherein Z is a fluorine atom or a trifluoromethyl group, m is an integer of from 0 to 3, p is 0 or 1, and n is from 1 to 12, provided that m+p>0.
A polymer electrolyte fuel cell is required to be operated under low humidity conditions where the relative humidity of the reaction gas (fuel gas and oxidant gas) is low, or under no humidity conditions, in order to simplify the fuel cell system or to reduce the cost.
However, the polymer (1) has low proton conductivity, and accordingly a polymer electrolyte fuel cell employing the polymer (1) as an electrolyte material of a catalyst layer has low power generation characteristics (such as output voltage) under low or no humidity conditions.
As an electrolyte material having high proton conductivity, the following polymer (2) has been proposed.
(2) A polymer having sulfonic acid groups (—SO3H groups) converted from —SO2F groups in a polymer having repeating units based on a perfluoromonomer having two —SO2F groups and repeating units based on TFE (Patent Document 1).
However, even a polymer electrolyte fuel cell employing the polymer (2) as an electrolyte material of a catalyst layer still has insufficient power generation characteristics (such as output voltage) under low or no humidity conditions.    PATENT DOCUMENT 1: WO2007/013533